1. Field of the Invention
The present invention relates to anti-thrombotic medical materials and methods for producing them, in which an atmospheric pressure glow discharge plasma treatment is applied to the surface of materials to form films on them. The action of the plasma on the surface produces an anti-thrombotic material.
2. Prior Art
Medical appliances made of plastic materials have been used widely and those used for general purposes include catheters, blood bags, blood transfusion sets, fluid transfusion sets and blood circulating devices for extracorporeal circulation. These appliances are primarily used as disposables and are generally made of thermoplastic materials such as polyethylene, polyethylene terephthalate, poly(vinyl chloride) (PVC), polypropylene and the like. Generally, these materials have excellent mechanical properties, but tend to lack biocompatibility. In particular, it has been found that the blood compatibility of these plastic materials is rather poor because they do not have any effective anti-thrombotic properties.
During past years, therefore, various efforts have been made for the purpose of improving blood compatibility by treating the surface of these macromolecule materials. A material can retain its original mechanical properties after surface treatment. Such efforts have given birth to interesting processes. Japanese Published Patent No. 168365/1986, for example, discloses a process in which a heparin-type anticoagulant is introduced on the surface of synthetic macromolecule materials by ionic bonds or covalent bonds. Another method involving immobilization of a urokinase-type anti-thrombotic agent also exists.
A surface treatment by low-temperature glow discharge plasma has been proposed, in which a blood compatible compound having anti-thrombotic properties is introduced on a treated base material by grafting or chemical vapor deposition (CVD). This glow discharge plasma method is known to be advantageous because only the surface of the material to be treated can be reacted without any pin-holes in the thin films on the surface. During this surface treatment, the physical properties of the bulk material are not made poorer.
In plasma treatment technology, however, currently only corona or arc discharge can occur at atmospheric pressure, but no glow discharge can be easily produced at atmospheric pressure. The glow discharge requires vacuum or reduced pressure. Thus, various modes of operation are used at vacuum conditions to perform the surface treatment continuously. For example, a roll of a material to be treated is set in a vacuum vessel batch-wise, treated with the glow discharge and then withdrawn continuously in the form of a sheet. There is also another method that involves a differential exhaust system, in which the pressure is gradually reduced from atmospheric to a vacuum. Japanese Published Patent No. 27536/1988. discloses this method which uses a sheet of a material in the surface treatment.
These continuous treatment methods, however, require comparatively large volume treatment apparatus as well as a vacuum pump of a great capacity for reducing pressure in the apparatus. Furthermore, fluctuation in the degree of vacuum has a significant effect on the quality of surface treatment so that the vacuum system should be strictly controlled. This makes it physically difficult to install a treatment apparatus of a large size since it must be held under such sensitive vacuum conditions. Other than the additional facilities necessary for establishing a high degree of vacuum, treatment costs may increase to an extent that it may be practically impossible to use this system for producing general-purpose plastic materials. Medical appliances manufactured from these general-purpose materials are often intended to be used as disposables, hence must be comparatively low cost items.
Furthermore, if the material to be treated is in the form of a soft tube, it is very difficult to treat its surface and it is nearly impossible to treat its inner surface, not to mention selective treatment of the surface.
The use of plastic tubes is currently wide spread in various fields. Therefore, the development of simpler, less costly and more widely applicable methods for reforming or treating the surface of plastic tubes is highly desirable.